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MCAT Question: Consumer or Predator?

In a small lake, sea urchins graze upon small grasses that grow on the bottom. After introduction of an exotic sea urchin, the small grasses became over grazed and absent. To solve problem, a species of otter is introduced into the lake to predate upon the sea urchins, and the balance between the urchins and sea grasses is restored. What position did the otter take in this? Keystone Predator

The otter would be a considered a Secondary consumer, the urchins are the primary consumers, and a hypothetical predator of the otter would be a tertiary consumer. Since the otter is responsible for preventing overgrazing of the small grasses by predating on the out of control urchin population, it is termed a keystone predator that exerts a “top-down” effect on the ecosystem.